In order to heat an environment or an object, different heating elements may be used. There are many forms of heating elements including water heaters, gas heaters, pressure heaters and electrical heaters.
For electrical heaters, the heating element commonly comprises a conductor over which a suitable voltage is applied. Consequently, a current flows through the conductor. The current is related to the voltage according to Ohm's law: U=R·I, wherein U is the provided voltage, I is the current and R is the total resistance of the heating element. The resulting power output which corresponds to an increased temperature of the heating element and thus, its surrounding environment, may be formulated as P=U·I=R·I2.
A common electrical heater is a heating cable comprising one or more electrical conductors. The total resistance of a heating cable is dependent on how the heating cable is constructed and also on the length of the heating cable. A shorter length results in a lower resistance and consequently a higher current under the condition that the voltage is kept relatively constant. As a consequence, the power output per unit length will increase with decreasing length and potentially reach dangerous levels. However, when arranging a heating cable, or any other elongated heating element, to a installation site, it is advantageously if the heating cable may be cut to a certain length.
For this problem, there are a number of known solutions. Parallel resistive heating cables, as for example disclosed in patent application U.S. Pat. No. 3,859,506, are designed for being cut to a length on an installation site for a specific application. In reality, this means that a parallel resistive cable is purchased as one very long cable and thereafter cut to length at convenience. This quality has made the parallel resistive cable an increasingly popular alternative to the commonly used series resistive cables which is not designed to be cut-to-length. Series resistive cable are instead provided with a fixed length which must be estimated from the intended application and specified on beforehand.
While a parallel resistive cable has advantages over a series resistive cable regarding the above mentioned quality, it has drawbacks regarding other qualities. Due to its construction, the parallel resistive cable is an expensive cable and is not as mechanically strong as a series resistive cable. Furthermore, a parallel resistive cable is generally limited in that it should have a length in the range of about 50-130 meters.
With heating cables having a long length, there is furthermore a risk of very high starting currents when a voltage is applied, in particular when the surrounding, and consequently the cable, has a low temperature. Therefore fuses with high rated current must be used, thus leaving the security questionable.
In order to overcome the risk of very high currents, the current may be set to a constant value, as disclosed in patent application U.S. Pat. No. 4,849,611. The application describes a heating cable comprising a resistive heating component and a temperature-sensitive component. The heater may be provided with a constant current or a constant voltage. However, the longer a heating element according to U.S. Pat. No. 4,849,611 is, the higher the required voltage will be since the current is kept constant. Thus, the security is still left questionable. Furthermore, an installation of such a cable cut-to-length for a specific application would be complex since the means for providing the required voltage would also need to be adapted to the specific application, i.e. be transformed from, e.g., a mains voltage into the required voltage which could differ several hundred volts. Thus, the installation would not only be complex but also costly.
It is desirable to overcome or reduce the above mentioned drawbacks while still retaining a possibility to adapt the length of the heating element at its installation site to a specific length required by a specific application.